Detector for combustible gas



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47r4e4/E VJ March 28, 1967 Filed April 30, 1965 M. ROBINSON DETECTOR FOR COMBUSTIBLE GAS 5 Sheets-Sheet 2 INVENTOR. MVa/v Z foam/501v arch 28, 1967 M. ROBINSON DETECTOR FOR COMBUSTIBLE GAS 3 Sheets-Sheet 5 Filed April 30, 1963 r n u u h n u u n u n W\\ n n u ll k F whh ww RN MN u5 mm A u w W) 1 \.\1P\ NV. n 555 n A u wi QM. \h\ E n L tan N h n I u u M? mm M u MQ n H m H n I n i Ii n m T {Iii}. kw H m H L1 IIIIIIIIIIIIIIII 1 l L United States Patent 3,311,455 DETECTOR FOR COMBUSTIBLE GAS Myron L. Robinson, Monterey Park, Califi, assignor to Barton Instrument Corporation, Monterey Park, Califi, a corporation of California Filed Apr. 30, 1963, Ser. No. 276,946 15 Claims. (Cl. 23-255) The present invention relates generally to detectors for combustible gases, such as hydrocarbon compounds in air, and more particularly to a detector utilizing catalytic combustion of the gas at a hot wire or filament having a catalytic surface.

When combustion takes place at the surface of a catalytic material the surface temperature increases. If this material has a temperature coefficient of resistance, its resistance will likewise change with temperature, with a positive coelficient an increase in resistance with an increase in temperature. If the catalytic material, in wire or filament form, for example, is placed in a sensitive Wheatstone bridge, the change in its resistance caused by the catalytic combustion of a detected gas may be readily observed.

In order to compensate for changes in ambient conditions, such as temperature, humidity, circulation and other variables, a filament substantially identical to the catalytic filament, except that its surface is not catalytic to the combustible gas, is preferably located as the reference resistance in the bridge circuit, and placed in the same environment as the catalytic filament and in such manner as to minimize effects of flow and convection Current is passed through the two filaments so as to raise their temperatures a like amount and to bring the highest temperature portion of the catalytic surface to a temperature at which catalytic combustion will take place in the presence of very small quantities of the combustible gas to be detected. The filaments are exposed to the air sample in the same manner so that they respond the same to all changes in sample composition and ambient conditions, except changes in the combustible gas content of the air sample.

Since the heat of combustion of the gas at the surface of the catalytic filament is added to the heat supplied by the current flowing therethrough, its temperature will, under conditions of constant current, rise to undesirable values which might cause burn out of the filament and ignition of explosive mixtures of gas. It is therefore desirable and a feature of the present invention to lower the current flowing through the resistive filaments, both catalytic and non-catalytic, in the presence of a combustible gas which establishes a temperature differential between the catalytic and non-catalytic filaments, Under the present invention, the current which heats the reference and catalytic filaments is also the current which operates the Wheatstone resistance bridge in which the filaments are disposed so that a single source of power is required and the reduction in current through the filaments and their temperature is secured by reducing the bridge voltage. The change in the bridge voltage indicates the concentration of combustible gas detected and measured by the instrument.

It has been found that combustion may be catalyzed at progressively lower temperatures as the concentration of combustible gas increases in the sample of air being investigated, and the bridge voltage is correspondingly lowered to lower the temperature of the filaments so that combustion on the catalytic filament is limited to that producing a desired imbalance in the bridge circuit. Thereby maximum protection of the filament from overheating and burn out is achieved and the detector is operable through the explosive range of concentration of 3,3ll,45 5 Patented Mar. 28, 1967 the combustible gas since the filament temperature will be kept below the ignition temperature of the explosive mixture. This enables the detector to be operated through a wide range of gas concentrations, including right through the explosive range.

It is therefore an object of the present invention to provide a detector for combustible gas using a heated filment having a catalytic surface which operates at a vari able temperature producing a controllable amount of catalytic combustion of the gas.

Another object of this invention is the provision of an improved detector for combustible gas which is portable, light in weight, continuously sampling, giving rapid respouse and of high sensitivity.

Another object of this invention is the provision of an improved detector for combustible gas employing a heated element having a surface which is catalytic for combus tion of the gas to be detected in which the temperature of the surface is decreased as the concentration of the gas increases to maintain the hottest portion of the element at the minimum temperature at which a desired amount of catalytic combustion occurs.

A further object of this invention is the provision of an improved detector for combustible gas employing a bridge circuit including comparison elements, one of which has a surface catalytic to combustion of the gas, in which the voltage across the bridge is reduced to a Value to secure minimum catalytic combustion and minimum unbalance of the bridge sufficient only to drive the bridge voltage to its reduced Value.

A still further object of this invention is the provision of a detector for combustible gas employing a bridge circuit including comparison elements, one of which has a surface catalytic to combustion of the gas, in which the voltage across the bridge is reduced to a value to secure minimum catalytic combustion and minimum unbalance of the bridge sufficient only to drive the bridge voltage to its reduced value, and having means selectively responsive to the bridge voltage with different sensitivity to provide different sensitivity measurement of the concentration of the combustible gas.

Yet another object of this invention is the provision of an improved detector for combustible gas employing a bridge circuit including comparison elements, one of which has a surface catalytic to combustion of the gas, in which the reference and sensitive elements are subjected to substantially identical exposure to the air being sampled to reduce variations caused by changes in ambient conditions.

Another object of this invention is the provision of an improved detector for combustible gas having an improved arrangement for continuous sampling of the air which the detector is to investigate.

These and other objects and features of the invention will be readily apparent to those skilled in the art from the following specification and the appended drawings, in which:

FIGURE 1 is a perspective view of a portable form of detector according to the present invention, as used;

FIGURE 2 is a longitudinal sectional view through the detector of FIGURE 1, with parts shown in elevation and other parts sectioned or broken away;

FIGURE 3 is a transverse sectional view through the detector cell of the detector instrument, on the line 33 of FIGURE 2;

FIGURE 4 is a View of the front end of the detector instrument, corresponding to the forward portion of FIGURE 2 enlarged;

FIGURE 5 is a detail sectional view taken at line 55 of FIGURE 4;

FIGURE 6 is a detail sectional view showing a valved outlet, taken on line 6-6 of FIGURE 4;

FIGURE 7 is a transverse sectional view at line 77 of FIGURE 4;

FIGURE 8 is a detail sectional view at line 8-8 of FIGURE 4; and

FIGURE 9 is a wiring diagram for the detector instrument according to the present invention.

The embodiment of the present invention selected for illustration in the drawings is a lightweight portable one, shown at 11 in FIGURE 1 as including a main body portion 12 from which depends a handle 13' by which the detector is shown held in the hand of the operator. The body portion 12 comprises a front plate 13 and an elongated, cup-shaped portion 14 cooperating therewith to form an enclosure for the detector parts. Through the back wall of the cupshaped member 14 project an instrument scale and needle 15, the handle 16 of a main power switch, and the manipulating handle 17 of an adjustable potentiometer in the comparison bridge, all of which will be described more fully in connection with FIGURE 9.

Projecting from the front of the detector 11 is a probe tube 18 through which air to be sampled is drawn from its tip portion 19 at the end thereof. The tip 19 is preferably end sealed and provided with a plurality of small, transverse, diffuser openings 21 through which air is sucked into the tube 18. The tube 18 is desirably formed in sections so that it may be disassembled for storing and includes an accessible pocket 22 within which may be placed a humidifying filter element 23 to which access may be had for re-wetting or replacement of the humidifying element. The filter element also functions to remove entrained water from an air sample.

The front plate 13 of the detector is provided with a generally centrally located opening 24 therethrough in which is press fitted or otherwise secured a projecting sleeve 25, exteriorly threaded at 26. Upon the front edge of the sleeve 25 is fitted a conical element 27 releasably retained thereon by a holding element 28 having a threaded portion 29 threaded on the portion 26 on the sleeve 25. The conical member 27 has a central bore 31 therethrough and an externally threaded nose 32 projecting therefrom. A nut 33 threads on the nose 32 and has a compressible washer 34 therein which is pressed against the tube 18 in sealing relation when the tube 18 is placed in the bore 31 and the nut 33 tightened. The probe tube 18 is thereby sealably mechanically coupled to the detector.

The inner face of the conical member 27 is countersunk at 35 to receive a bafiie disc 36 having a plurality of notches 37 about its periphery through which the gas sample must pass, the gas being thereby prevented from traveling in a straight line in the sleeve 25 as it leaves the tube 18.

Interiorly of the sleeve 25, adjacent the back end thereof, is disposed an insulating ring 38 and within the ring 38 is disposed a second insulating ring 39. About the forward edge of the insulating ring 39 are disposed three circumferentially spaced, stationary connector members or contacts 41, having contact portions 42 facing forwardly, and connector legs 43 disposed in notches in the periphery of the ring 39 and held between the rings 38 and 39. A circumferential notch 44 is disposed in the ring 39 to receive circuit wiring 45 connected to the connector legs 43.

Within the sleeve 25, forwardly of the insulating rings 38 and 39, is mounted a detector cell 51. The cell 51 includes both the catalytic and inert filaments and the connections thereto, and is provided with means for controlling fiow of the air samples into contact with both the reference and active filaments. It functions addition-' ally as a flame arrester. The detector cell 51 includes an outer insulating ring 52 having a central bore 53 therethrough and countersunk recesses 54 at its opposite faces. The insulating ring 52 is provided with three circumferentially spaced, radially directed bores therethrough in 4 which are disposed the elongated legs 55 of generally L-shaped connectors 56. The connectors 56 have rear-- wardly facing contact portions 57 engageable with thecontact portions 42 on the stationary connector mem-- bers 41.

Adjacent the ends of two of the connector legs 55 are: mounted the ends of a filament 58, as by welding or brazing. To substantially the mid-point of the filament 58 is welded or otherwise secured the third connector leg 55.. This mid-point is shown in FIGURES 3 and 9 at 59.- The filament 58 is thereby electrically divided into two half portions 61 and 62. The filament 58 is preferably constructed so that one half 61 thereof provides a surface catalytic to combustion of the gas to be detected, while the half 62 has a non-catalytic surface, with the halvesof the filament otherwise identical. In one embodiment of the invention the filament 48 may be a platinum wire with the catalytic half 61 thereof uncoated, while the half 62 has deposited thereon an extremely thin coating: of silver or lead which may be of little more than molecular thickness but which effectively poisons the surface of the half 62 so that it is not catalytic to combustion of the gas. If the two halves of the filament do not give desired non-catalytic response, the mid-point 59 may be slightly shifted to compensate.

The insulating ring 52 and sleeve 25 may be provided with a cooperating rib and groove at 63 by which the detector cell is indexed in the sleeve to insure mating of the contact portions 42 and 57 to give electrical connection to the filament 58. The detector cell 51 is held in place, with the contact portions 42 and 57 tightly engaged, by the engagement therewith of a resilient sealing washer disposed between a cap 72 (to be later described) and the conical member 27, the latter being clamped in place by the member 28 threaded on the sleeve 25.

The filament 58 is embedded between two porous, Y- shaped mats of silica or quartz fibres which totally encase and support the filament 58 but through which the gas to be sampled may flow into contact with the surfaces of the filament. These quartz fiber mats are designated by the numeral 64 and are spaced from the ring 52 at 65 to provide for free passage of a portion of the air sample therethrough to pass directly to the pump and exhaust without contacting the filaments. At opposite sides of the quartz fibres 64, and in the recesses 54 in the opposite faces of the insulating ring 52, are mounted flow distribution plates 66 and 67 held on the insulating ring 52, with the quartz fibres therebetween, by a through bolt 68. The plates 66 and 67 may be formed of sintered refractory or metallic material, such as bronze, to provide for uniform passage of the air sample therethrough. It will be apparent that the detector cell 51 may be utilized as well in a diffuser system without positive flow therethrough.

In front of the plate 67, a pair of filter discs 69 and 71 are mounted on the insulating ring 52 by a clamping cap 72 mounted against the front face of the ring by studs 73. The cap 72 has a large central opening 74 therethrough for the passage of air through the detector cell. The filters 69 and 71 may be of any desired mechanical or chemical nature, and in a preferred embodiment of the invention, the filter 71 is a low air flow resistant glassfibre mat while the filter 69 is formed of Teflon fibres or other non-wetting material to insure that water is kept out of the cell. With the filtering arrangement shown, it will be seen that all suspended material will be removed from the air to be sampled; and with the porous plate and quartz fibre arrangement, the air will travel in a linear flow fashion through the detector cell, principally through the openings 65 but with a portion of the air moving gently across and in contact with the filament 58 through the quartz fibres 64 to provide an air sample in contact with the filament for the rapid detection of the presence of combustible gas therein.

II a

A valve plate 31 in the form of a second insulating plate is mounted in back of the front plate 13, as by studs 82. The valve plate 81 has a central opening 33 therethrough communicating with the rear porous plate 66 on the detector cell through the insulating ring 39. The back face of the valve plate 81 is recessed at 84 to receive a disc 85 rigidly pressed therein and provided with a central opening 36 therethrough. A flapper 87 of soft, flexible sheet material is mounted on the back face of the disc 85, as by cementing, at 38, and positioned so as to close and open the opening 86 in valving relation.

A rectangular frame 89 is mounted on the back of the valve plate 81, as by studs 91, and between the frame 39 and the plate 81 is disposed a flexible, elastic sheet 92, which may be of rubber-like material, thereby sealed against the back face of the valve plate 81 to provide a pump chamber therebetween.

The front plate 13 has a threaded bore 70 therethrough at the lower portion thereof which is closed by a flush screw 76. An opening 77 through the valve plate 81 connects the bore 70 with the pump chamber defined between the back surface of the valve plate 81 and the flexible sheet 92. A valve flapper 78 of soft, flexible material is mounted on the front face of the valve plate 81 within the bore 71, as by cementing at 79, in position to close and open the opening 77 to control the passage of air therethrough. An exhaust slot 30 in the back face of the front plate 13 extends from the bore 70 to the exterior of the detector to pass sampled air out of the instrument.

Over the major area of the flexible sheet 92 is cemented or vulcanized a plate 93 of magnetic material forming an armature attractable to a stationary magnetic core 94 energizable by an electrical coil 95 therearound. A path for return flux is provided by a U-shaped magnetic member 96 about the coil 95. The end of armature 93 is supported against the upper leg of member 96 for pivoted movement toward and away from the core 94 by the flexible, elastic sheet 92, and its movement expands and contracts the pump chamber. A post 97 is adjustably mounted in the valve plate 81 and has a reduced neck 98 adjacent its inner end which is received Within a slot 99 in the lower end of a spring strip 101 (FIGURES 4 and 8). The upper end of the spring strip 101 is rigidly secured to the bottom of the armature plate 93 by a rivet 102. Adjustment of the position of the post 97 forwardly and rearwardly of the plate 81 will determine the deenergized position of the armature plate 93 and therefore the extent and frequency of its pump operating stroke.

Mounted beneath the Ushaped member 96 is an electrical switch 103 of normally closed contact construction and having an operating plunger 104- positioned to be engaged by the rivet 102 or some other part movable with the armature plate 93 to effect opening of the contacts of the switch 103 prior to the completion of the full attracted movement of the armature plate. The strength of the return spring for the plunger 104 also influences the pump cycling frequency.

A supporting bracket 105 is rigidly secured to the frame 89, as by bracket 106 and studs 107. The bracket 105 has superposed horizontal portions 108 and 109 and a vertically extending rear portion 111. Upon the lower horizontal portion 108 is mounted an electrical component board 112 and upon the upper horizontal portion 109 is mounted a meter 113 having a scale and needle projecting into view through the base of the outer cu-pshaped enclosure member 14. Upon the vertical section 111 of the bracket 105 are mounted a switch 114 and a potentiometer 115 whose operating handles 16 and 17, respectively, were referred to in the description of FIG- URE 1. p

The handle 13' of the instrument is connected to a rigid plate 116 which is in turn connected to the bracket 105 by a reduced neck 117 received in a bottom slot 118 in the outer enclosing cup 14. Within the handle 13' is mounted an electrical switch 119 having an operating button 121 projecting into position to be engaged by the index finger of an operator holding the handle 13'. The electric switch 119 has sequentially operating, normally closed and normally open contacts therein, with the normally closed contacts opening upon initial movement of the pushbutton 121 and the normally open contacts closing upon further movement of the pushbutton 121, the normally closed contacts remaining open. A battery source of power for the instrument may be carried within the instrument or a separate power source may be connected through the conductor leading through the handle 13'.

A wiring diagram for the detector according to the present invention is illustrated in FIGURE 9, and may be divided roughly into three sections: a sensing and control section 125, an output indication section 126 and a pump and voltage regulator section 127. A battery source is shown at 128 which may be of any desired voltage, for example, 5 volts, and is connected to main lines 129 and 131. The filament halves 61 and 62 are disposed in the legs of a Wheatstone bridge arrangement connected across line 129 and a sub-line 132. The mid-point 59 between the filament halves 61, 62 becomes a measuring point across the bridge. The other legs of the Wheatstone bridge are formed by fixed resistors 133 and 134 and the adjustable resistor or potentiometer 115, previously referred to. The adjustable point 135 on the potentiometer 115 provides the other measuring oint across the Wheatstone bridge. A transistor, or equivalent electronic .amplifying element 136 has its base connected to the point 135, its collector to the point 159, and its emitter to a series of emitter-follower connected transistor amplifiers 139 and 141 and transistor 142. The transistor 14-2 operates to control the voltage across the bridge by the variable voltage drop across the transistor, its emitter and collector being between sub-line 132 and line 131, respectively. The emitter of transistor 136 is connected through a thermistor 137 to sub-iine 138 to lessen the effect of temperature variation on the control circuit. This thermistor corrects for variation in transistor characteristics caused by temperature changes.

The energizing coil 95 is connected across the lines 129 and 131 through the normally closed switch 103. It is paralleled by a rectifying diode 1 13 and a blocking diode 144 and their junction is connected to a filter circuit containing capacitor 145 and resistor 146, which are serially connected across line 129 and sub-line 138. Also connected between line 129 and sub-line 138 is a voltage regulating Zener diode 147 whose Zener voltage is preferably approximately equal to normal battery voltage.

Across the line 129 and sub-line 138 is mounted a voltage dividing potentiometer 143 whose adjustable takeoff point 149 is connected through a sensitivity reducing resistor 151 to the negative terminal of the meter 113. The positive terminal of the meter 113 is connected through the collector-emitter of a transistor 152 to the sub-line 132, the base of the transistor 152 being connected to the adjustable take-off point 149 of the potentiometer 148. The positive side of the meter 113 is also connected through a diode 153 and the normally open contacts 154 of switch 119 to the sub-line 132. The negative side of the meter 113 is connected through a capacitor 155 and the normally closed contacts 156 of switch 119 to the sub-line 132. The positive side of the meter 113 is also connected by a resistor 157 to the line 129.

The operation of the detector of this invention will now be described: In using the detector the tip 19 will be placed in localities Where it is suspected that combustible gas may exist. For example, it may be moved along the top of a pipeline in the field or may be placed adjacent the supply, or metering or consumption devices in buildings. It may likewise be used in mines and other locations where combustible gas may be present in the air.

With the embodiment of the invention illustrated in the drawings, the humidifier element 23 may or may not be used in operating the device. Where used, it constitutes a holder for moisture, such as a water absorptive filter, which will give substantially uniform high humidity to the air being sampled through the detector. Its use is influenced by the fact that if the air is always at uniform high humidity when passed through the detector cell, the instrument will be substantially unaffected by changes in humidity of the air being sampled.

In readying the detector for operation, the switch 114 is closed to connect the energizing coil 95 across the lines 129 and 131 to energize the pump provided by the armature 93 and the flexible sheet 92 on which it is mounted. Closing of the switch 114 also energizes the Wheatstone bridge to pass current through the filament 58. In setting up the initial adjustment of the instrument, the potentiometer 115 is adjusted to give the desired voltage a'cross he Wheatstone bridge circuitthat is, across the line 129 and sub-line 132so that the active element or catalytic half filament 61 is brought to a temperature at which catalytic combustion of gas will occur for relatively small concentrations of such gas. This temperature is not critical, but when testing, for example, for methane, the temperature of the central portion of the catalytic filament half should run at the order of about 600 700 C. It will be understood that the filaments 61 and 62 will not be of uniform temperature throughout, as the heat generated therein by the passage of current therethrough is rapidly led therefrom by the connector legs 55 and only the center portion of each half filament will be at its maximum temperature, and the extent of the active central area determines the amount of catalytic combustion which will occur on the surface of the active element 61. With the potentiometer 115 adjusted to give the desired voltage across the bridge and the desired operating temperature for the active element or filament half 61, the potentiometer 148 is adjusted to give about a 20% reading on the sensitive scale of the meter 113, that is, with the switch 119 not operated. The selection of such an upward point upon the scale for the adjustment of the potentiometer 148 will be explained hereinafter. After the potentiometer 148 is once set, it is left fixed, and all adjustments thereafter are made by means of the potentiometer 115.

When the coil 95 is energized, it establishes a magnetic field in the core 94 which attracts the armature 93. As the armature 93 moves toward the pole face of the core, the pump chamber between the flexible sheet 92 and the back face of the plate 81 increases in volume to lower the pressure therein below atmospheric and create a suction which tends to draw air from the tube 13 through the detector cell 51. Before the armature 93 engages the pole face of the field core, it opens switch 103 by engagement with the pushbutton operator 1&4 of the switch so the coil 95 is deenergized and the armature 93 tends to return toward its unattracted position. However, in this movement, the pushbutton 194 moves with the armature 93 'and the switch 193 again closes to reenergize the coil 95 so the armature is re-attracted towards the core and the same procedure occurs over and over again very rapidly, with the armature being attracted 'and returning toward its unattracted positions in response to the repeated energization and deenergization of the coil 95 as the switch 103 is opened and closed by movement of the armature.

This armature movement moves the flexible sheet 92 so that the volume of the pump chamber between the sheet and the back face of the plate 81 is rapidly increased and decreased by the rapid flutter movement of the ar. .ature and the flexible sheet on which it is supported. When the volume of the pump chamber is increasing, the flapper 3'7 opens freely to open the hole 86 and permit air to enter the chamber from the tube 18. At this time the flapper 78 is closing the opening 77, so air is prevented from moving back through the exhaust outlet into the pump chamber. When the volume of the pump chamber is decreasing, the flapper 8'7 closes the opening 26 to prevent air being forced in the reverse direction into the tube 18, and at this time the flapper 78 opens, so that the air in the pump chamber is exhausted through the opening 80. Therefore, the rapid oscillations of the flexible sheet 92 serve to pump air from the tube 18 through the detector cell 51 and the pump to the outlet 39. The detector cell construction provides stability of operation with pulsing pump flow.

The air enters the tube 18 through the diffuser openings 21 in its tip 19 and passes through the humidifier 23, where used, to give a uniform high humidity to the 'air to be sampled in the instrument. The air passes from the tube 13 through the peripheral outlets in the baffle disc 36, through the filtering glass fiber mat 71 and the filtering Teflon fiber pad 69, to the flow distributing plate 67. The air passing between the porous plates 67 and 66 will in large part pass through the free openings 65 adjacent the interior surface of the ring 52, and the portion thereof passing through the silica fibers 64 and into contact with the filament 53 will move relatively slowly through the material into contact with the filament surfaces. After passage through the plate 66, the air will pass through the opening 86, the pump chamber, the opening 77, the bore 70, to the outlet 80. With the operation of the pump previously described, there is substantially continuous sampling effected of the air into which the tip 19 of the probe tube 18 is projected. With this continuous sampling, it will be seen that there is relatively rapid response of the instrument to the projection of the tip 19 into an 'air volume, since the pump clears the tubing 18 of air relatively rapidly, even while the sample is moving relatively slowly through the silica fibers past the tilement 58.

Referring now to FIGURE 9 for the wiring diagram for the detector of this invention, the voltage across lines 129 and 131 will be the voltage supplied by the battery 128. The voltage across the lines 129 and 132 is the voltage across the Wheatstone bridge and this voltage is the battery voltage less the voltage drop through the power transistor 142. The voltage of the line 132 is thus varied with the concentration of combustible gas encountered so as to lower the bridge voltage to lower the current passing through the element halves 61 and 62 of the filament 53.

The voltage across the line 129 and sub-line 133 is held substantially constant to the Zener voltage of the Zener diode 147, which is preferably approximately normal battery voltage. This is accomplished by adding to the actual battery voltage the rectified, back-induced voltage from the energizing, coil 95. Each time coil is interrupted, a back is generated therein which is rectified, filtered by the capacitor-resistor combination 145, 146, and added to the battery voltage, with the added voltages regulated to a substantially constant value by the Zener diode 147. This Zener regulated voltage appears across the line 129 and sub-line 13S and insures operation of the instrument as the battery voltage declines from extended use. It thus functions to extend effective battery life and to provide a reference voltage for the meter and maintain constant voltage for transistors 139 and 141.

Assuming the tip 19 of the tube probe 18 of the instrument is in air with no detectable amount of combustible gas present, the transistor 136 is driven by the minimum unbalance of the bridge which is necessary to drop the voltage at sub-line 132 (by the voltage drop through transistor 142) to the point where the current through the filament elements 61 and 62 is just snfiicient to heat the element 61 to the point where its active portion will effect detectable catalytic combustion of very small concentrations of the gas to be detected. This operation is effected through the emitter-follower amplifier transistors 9 139 and 141 to Control the voltage drop through the transistor 142, and hence the voltage at the sub-line 132 and the voltage across the Wheatstone bridge.

For searching for the presence of combustible gas, the meter 113 is on its most sensitive scale, with the switch 119 unoperated and the circuit components in the positions shown in FIGURE 9. As the tip 19 of the probe enters a gas pocket, catalytic combustion occurs at the filament half 61 to initially increase its temperature and resistance. This increases unbalance of the bridge so that the amplifier transistor 136 supplies a signal to increase the voltage drop through the transistor 142 and thereby lower the voltage across the bridge. This decreases the current through the filament 58 to lower the temperature of both filament halves and decrease catalytic combustion. Where the filament is operated initially at a high temperature for very high sensitivity, the initial lowering of voltage, current and temperature may not significantly affect catalytic combustion. The change in voltage at the line 132 is amplified through transistor 152 to appear at collector connected terminal of meter 113. An instantaneous current flows through the meter to change capacitor 155 to the new voltage level and Will cause the meter needle at 15 to swing out of its stable position. The meter will not remain in an indicating position, but will return to its initial preset position as the capacitor charging current declines. When the probe tip 19 leaves the same pocket of gas, the temperature of the active or catalytic element 61 decreases to decrease bridge unbalance so that transistor 136 functions in the opposite direction to decrease the drop through transistor 142 and to raise the voltage of the sub-line 132. This voltage increase causes an instantaneous current flow through the meter 113 in the opposite direction as the charge on capacitor 155 is adjusted to the new voltage and the needle at 15 swings in the reverse direction. Setting the needle initially at about 20% of scale provides for drift which may occur on the sensitive scale and permits observation of downscale movement as gas concentration decreases. It is to be noted that resistor 151 is not included in the same meter circuit on the sensitive range, but is shunted by capacitor 155 to increase sensitivity without increase in drift.

The concentration of gas in a pocket can be estimated from the amount of swing of the meter needle and on the sensitive search scale the instrument can be made responsive to detect about parts of gas in 1,000,000 parts of air, such a minute concentration giving a measurable needle swing.

When a gas pocket has been located, the detector can be operated to give a fixed reading for the concentration, if suificient gas is present. For this purpose, the switch 119 is operated by partially depressing the pushbutton 121 to open the contact 155 while the contact 154- remains open. The meter 113 will now be placed on intermediate scale to read the output of transistor 152 through scale coarsening resistor 151. Variation in the voltage of the sub-line 132 will vary the bias across the emitter-base of the transistor 152 and its correspondingly varying output is read by the meter 113. The output is seen to be continuous so that the registering needle and scale at assume a relative position corresponding to the concentration of gas in the air. The meter reading may be interpreted either directly or by calibration charts which indicate the concentration of gas present.

Should the concentration of gas be too great for intermediate scale operation of the meter 113, the pushbutton 121 of switch 119 is pressed entirely in to close contact 154, with contact 156 remaining open. This places a direct connection to the meter 113 through the switch 154, shunting transistor 152. The voltage of sub-line 132 is no longer amplified through transistor 152 and the meter operates on coarse scale for reading maximum concentrations of gas.

The operation of the sensing and control bridge of the detector according to the present invention is such as to run the catalytic filament half 51 always at a temperature which will produce only su'ificient catalytic combustion to unbalance the bridge only sufiiciently to cause the adjusting circuitry to drive the bridge voltage to the value at which such amount of catalytic combustion takes place. This means that the hottest portion of the catalytic filament 61that is, the mid-portion of this half of the filament-is operating at progressively lower temperatures as the concentration of gas increases. By this operation, the hottest part of the catalytic filament half 61 is main tained below the ignition temperature of explosive mixtures of combustible gas which occur, for example, substantially in the range of 5% to 20% of methane in air. Where the catalytic element is operated at temperatures above the ignition point, obviously the instrument cannot be used for gas concentrations in the explosive range, but the instrument according to the present invention operates at so low a temperature that it can be and is used right through the explosive range of gas concentration.

The potentiometer is adjustable from the exterior of the instrument by the operating handle 17 to compensate for changes in filament characteristics through the life of the filament, and for other circuit characteristics to return the meter to substantially 20% of scale at no gas present.

It is therefore seen that the detector according to the present invention lends itself to a light-weight portable construction, provides for automatic and continuous sampling, and rapid response to the presence and change in concentration of a combustible gas in the air. It operates with low current and power consumption and gives high sensitivity to small gas concentrations of the order of parts per million parts in air. hottest part of the catalytic filament only slightly above the temperature of catalytic combustion, the filament is kept below the ignition temperature when exposed to explosive mixtures of gas and the instrument reads right through the explosive range and on up to higher concentrations of gas. This low tem erature operation also gives a high degree of differentiation between gases of different compositions. By exposing the filament halves to the air sample in the same manner, compensation for all variables in the sample is quite good and reliable measurement of the combustible concentration can be made.

While certain preferred embodiments of the invention have been specifically illustrated and described herein, it will be understood that the invention is not limited thereto as many variations will be apparent to those skilled in the art, and the invention is to be given its broadest interpretation under the terms of the following claims.

I claim:

1. In a detector for detecting varying concentrations of combustible gas including concentrations in the explosive range, a bridge circuit having active and inactive elements in legs thereof, said elements being substantially identical except that the active element only has a surface catalytic to combustion of the gas to be detected, said elements being connected in the bridge so that the voltage across the bridge passes identical current through the elements whose value varies with variation in the voltage across the bridge, said elements each having a substantial temperature coefiicient of resistance so that their resistances vary with their temperature; means for placing an air sample in contact with both of said elements under substantially identical conditions whereby the presence of combustible gas in the sample will cause catalytic combustion at the surface of said active element to raise its temperature and effect a difference in the temperature and thus the resistance of said elements which will cause unbalance of the bridge; and means responsive to unbalance of the bridge to lower the voltage across the bridge and thereby reduce the current through By operating with the said elements to a value producing at any concentration of combustible gas throughout the range of operation of said detector the minimum amount of said catalytic combustion which will cause only sufiicient difference between the temperatures of the elements to unbalance the bridge only sufliciently to secure the minimum signal which will drive said bridge voltage to said lowered value.

2. The detector defined in claim 1, in which the resistances of the legs of the bridge other than the leg including said elements are relatively adjustable to establish a bridge unbalance for the original setting of the voltage across the bridge and the current through the elements to raise the temperature thereof to the point at which catalytic combustion of a gas to be detected will occur at the hottest surface of the active element.

3. In a detector for detecting varying concentrations of combustible gas including concentrations in the ex plosive range, a bridge circuit having active and inactive elements in legs thereof, said elements being substantially identical except that the active element only has a surface catalytic to combustion of the gas to be detected, said elements being connected in the bridge so that the voltage across the bridge passes identical current through the elements whose value varies with variation in the voltage across the bridge, said elements each having a substantial temperature coeflicient of resistance so that their resistances vary with their temperature; means for placing an air sample in contact with both of said elements under substantially identical conditions whereby the presence of combustible gas in the sample will cause catalytic combustion at the surface of said active element to effect a difference in the temperature and thus the resistance of said elements which will cause unbalance of the bridge; means responsive to unbalance of the bridge to lower the voltage across the bridge and thereby reduce the current through said elements to a value producing at any concentration of combustible gas throughout the range of operation of said detector the minimum amount of catalytic combustion which will cause only sufficient difference between the temperatures of the elements to unbalance the bridge only sufiiciently to secure the minimum signal which will drive said bridge voltage to said lowered value; said last mentioned means including an electronic component in series with the bridge having a variable voltage drop thereacross; and means responsive to unbalance of the bridge for varying the voltage drop across said electronic component to vary the voltage across the bridge.

4. The detector as claimed in claim 3, in which said electronic component comprises a transistor; and means for varying the forward bias of said transistor in accordance with the unbalance signal from the bridge so as to vary the voltage drop across the transistor.

5. In a detector-for detecting varying concentrations of combustible gas including concentrations in the explosive range, a bridge circuit having active and inactive elements in legs thereof, said elements being substantially identical except that the active element only has a surface catalytic to combustion of the gas to be detected, said elements being connected in the bridge so that the voltage across the bridge passes identical current through the elements whose value varies with variation in the voltage across the bridge, said elements each having a substantial temperature coefiicient of resistance so that their resistances vary with their temperature; means for placing an air sample in contact with both of said elements under substantially identical conditions whereby the presence of combustible gas in the sample will cause catalytic combustion at the surface of said active element to raise its temperature and effect a difierence in the temperature and thus the resistance of said elements which will cause unbalance of the bridge; means responsive to unbalance of the bridge to lower the voltage across the bridge and thereby reduce the current through said elements to a value producing at any concentration of combustible gas throughout the range of operation of said detector the minimum amount of said catalytic combustion which will cause only sufl'icient difference between the temperatures of the elements to unbalance the bridge only sufficiently to secure the minimum signal which will drive said bridge voltage to said lowered value; a capacitor; means responsive to variation in the voltage across the bridge for varying the charge on said capacitor; and a meter in circuit with the capacitor for indicating by its swing the instantaneous current to the capacitor.

6. The detector as defined in claim 5, including: an alternate circuit for said meter responsive to the voltage across the bridge; means for connecting said meter in said alternate circuit to provide a steady reading on the meter on a coarser scale for greater concentrations of combustible gas in the air sample, said connecting means including means for opening said capacitor circuit for operation of said meter at a steady position on said greater gas concentrations.

7. The detector as claimed in claim 6, including means [optionally connectible in said alternate meter circuit for amplifying the change in bridge voltage to provide a finer meter reading for intermediate ranges of gas concentration.

8. In a detector for detecting varying concentrations of combustible gas including concentrations in the explosive range, a bridge circuit having active and inactive elements in legs thereof, said elements being substantially identical except that the active element only has a surface catalytic to combustion of the gas to be detected, said elements being connected in the bridge so that the voltage across the bridge passes identical current through the elements whose value varies with variation in the voltage across the bridge, said elements having a substantial temperature coefficient of resistance so that their resistances vary with their temperature; means for placing an air sample in contact with both of said elements under substantially identical conditions whereby the presence of combustible gas in the sample will cause catalytic combustion at the surface of said active element to raise its temperature and effect a difference in the temperature and thus the resistance of the elements which will cause unbalance of the bridge; means responsive to unbalance of the bridge to lower the voltage across the bridge and thereby reduce the current through said elements to a value producing at any concentration of gas throughout the range of operation of said detector the minimum amount of said catalytic combustion which will cause only suflicient difference between the temperatures of the elements to unbalance the bridge only suli'iciently to secure the minimum signal which will drive said bridge voltage to said lowered value; a capacitor; means responsive to variation in the voltage across the bridge for varying the charge on a capacitor; and a meter in circuit with the capacitor for indicating by its swing the instantaneous current to the capacitor, said meter circuit including amplifying means for amplifying the change in bridge voltage to increase the sensitivity of the meter.

9. In a detector for combustible gas, a bridge circuit having active and inactive elements in legs thereof, said elements being substantially identical except that the active element only has a surface catalytic to combustion of the gas to be detected, said elements being connected in the bridge so that the voltage across the bridge passes identical current through the elements whose value varies with variation in the voltage across the bridge, said elements having a substantial temperature coefilcient of resistance so that their resistance varies with their temperature; means for placing an air sample in contact with both of said elements under substantially identical conditions whereby the presence of combustible gas in the sample will cause catalytic combustion at the surface of said active element to raise its temperature and effect a difference in the temperature and resistance of said elements which will cause unbalance of the bridge; means responsive to unbalance of the bridge to lower the voltage across the bridge and thereby the current through said elements to a value producing the minimum amount of said catalytic combustion which will cause only sufficient difference between the temperatures of the elements to unbalance the bridge only sufiiciently to secure the minimum signal which will drive said bridge voltage to said lowered value; a battery source of power for the voltage across the bridge; a pump for supplying said air sample including an energizing coil therefor connected and disconnected to said battery source in rapid succession and with the disconnection of said energizing coil producing aback therein; means for rectifying said back and adding it to said battery voltage; and a voltage regulator for regulating the value of said combined voltages.

10. In a detector for combustible gas, detection means including an element having a surface catalytic to combustion of the gas to be detected; a direct current power supply for passing current through said element to heat the same; means responsive to catalytic combustion of gas at the surface of said element to indicate the presence of said gas; pump means for passing air samples across said element to detect the presence of gas therein; said pump means including an energizing coil and means for rapidly energizing and deenergizing said energizing coil from said power supply to perform the pumping operation; means for rectifying the hack generated in said coil when the current therethrough is interrupted and adding it to the voltage of said power supply; and a voltage regulator for regulating the combined voltages to a standard value to stabilize an operating voltage to be independent of variations in the voltage of said power supply.

11. In a detector for combustible gas, detection means including an element having a surface catalytic to combustion of the gas to be detected; a direct current power supply for passing current through said element to heat the same to catalytic combustion temperature; indicating means responsive to combustion of gas at the surface of said element for indicating the existence of the gas in an air sample; pump means for passing air samples across said element to detect the presence of gas therein, said pump means including an energizing coil and means for rapidly energizing and deenergizing said coil from said power supply to perform the pumping operation; means for rectifying the back E.M.F. generated in said coil when the current therethrough is interrupted and adding it to the voltage of said power supply; and a Zener diode connected across said combined voltages to regulate them to a standard value independent of variations in the voltage of the power supply.

12. In a detector for combustible gas, detection means including an element having a surface catalytic to combustion of the gas to be detected; a direct current power supply for passing current through said element to heat the same to catalytic combustion temperature; indicating means responsive to combustion of gas at the surface of said element for indicating the existence of the gas in an air sample; pump means for passing air samples across said element to detect the presence of gas therein; said pump means including a pump chamber defined in part by a sheet of flexible material whose movement increases and decreases the volume of the pump chamber to effect the pumping operation; an armature for said pump means mounted on said flexible sheet to effect movement thereof; a stationary magnetic structure toward which said armature is attracted; an energizing coil for said stationary magnetic structure; means for rapidly connecting and disconnecting said energizing coil to said power supply to effect rapid movement of the armature and the flexible sheet as the armature is moved between attracted and unattracted positions to perform the pumping operation; means for rectifying the back generated in said energizing coil when the current therethrough is interrupted and adding it to the voltage of said power supply; and a voltage regulator connected across said combined voltages for 14 regulating them to a standard value independent of variations in the voltage of said power supply.

13. In a detector for detecting varying concentrations of combustible gas including concentrations in the explosive range, active and inactive elements substantially identical except that the active element only has a surface catalytic to combustion of the gas to be detected; means for passing electrical current through said elements to raise them to substantially identical temperatures at which catalytic combustion will take place at the surface of the active element in the presence of combustible gas; means for flowing air samples in contact with said elements under substantially identical conditions so that any difference in their temperatures and thus in their resistances will be due solely to the catalytic combustion of gas at the surface of the active element, and means responsive to a difference in the resistance of said elements for lowering the current therethrough to the minimum value which will produce catalytic combustion only sufiicient to maintain a difference between the resistances of the elements which difference is the minimum value required to control said current, said catalytic combustion being incomplete even in the presence of an explosive mixture of air and said combustible gas.

14. In a detector for combustible gas, a filament of a material having a substantial temperature coefficient of resistance; means providing electrical connection adjacent the ends and at substantially the mid-point of said filament to divide it electrically into substantially identical half-portions; means rendering the surface of one half-portion of said filament catalytic to combustion of a gas to be detected and the surface of the other half-portion inactive; means for passing current through said filament to electrically raise the temperatures of said half-portions identical amounts and to raise at least the hottest portion of said catalytic half-portion to a temperature to effect combustion of gas present in an air sample in contact therewith; fibrous embedding means about said filament for supporting and controlling the flow of an air sample thereacross; means for drawing air samples through said fibrous embedding means in a relatively gentle flow to expose said half-portions of the filament to the air samples under substantially identical conditions so that the difference in temperature and resistance of the half-portions will be due substantially solely to the combustion of gas at the surface of the catalytic half-portions; a probe projectable into location from which an air sample is to be taken and leading to said filament; and passages bypassing said fibrous embedding means and filament to provide for withdrawal of relatively large volumes of air through the probe while gently fi-owing a small volume sample across the filament so as to secure a rapid response to change in probe location.

15. In a detector for combustible gas, a filament of a material having a substantial temperature coefiicient of resistance; means providing electrical connection adjacent the ends and at the mid-point of said filament to divide it electrically into substantially identical halfportions; means rendering the surface of one half-portion to said filament catalytic to combustion of a gas to be detected and the surface of the other half-portion inactive; means for passing current through said filament to electrically raise the temperatures of said half-portions identical amounts and to raise at least the hottest portion of said catalytic half-portion to a temperature to effect combustion of gas present in an air sample in contact therewith; an inert, porous, fibrous material in which said filament is embedded; flow distributing means in front of said fibrous material; means for drawing air samples through said flow distributing means and porous material across said filament so as to subject the halfportions of the filament to the air samples under substantially identical conditions so that the difference in temperature and resistance of the half-portions will be 15 15 due substantially solely to the combustion of gas at the 2,363,478 11/ 1944 Boeke 23-255 XR surface of the catalytic half-portion; a probe through 2,400,923 5/1946 Farr 7327 XR which air samples are drawn, to said filament; and pas- 2,441,677 5/ 1943 st ml h 3255 XR sages for the air samples bypassing said porous embed- 2,687,342 8/1954 slrange 23255 XR ding material and filament to provide for drawing a 5 2,245,721 5/1256 TlfiaIlY relatively large volume of air through said probe while 2,505,134 9/1727 Strange 'gently flowing a small volume sample across the fila- "Egg; as to Secure a rapld response to change 1n probe 2:926:520 3/1960 schmauch 73 27 10 3,232,712 2/1966 Stearns 23255 References Citefl by the Examiner MORRIS O. WOLK, Primary Examiner.

UNITED STATES PATENTS JOSEPH SCOVRONEK, Assistant Examiner. 1,995,370 3/1935 Stone 23255 2,204,966 6/1940 Morgan 23-255 15 

1. IN A DETECTOR FOR DETECTING VARYING CONCENTRATIONS OF COMBUSTIBLE GAS INCLUDING CONCENTRATIONS IN THE EXPLOSIVE RANGE, A BRIDGE CIRCUIT HAVING ACTIVE AND INACTIVE ELEMENTS IN LEGS THEREOF, SAID ELEMENTS BEING SUBSTANTIALLY IDENTICAL EXCEPT THAT THE ACTIVE ELEMENT ONLY HAS A SURFACE CATALYTIC TO COMBUSTION OF THE GAS TO BE DETECTED, SAID ELEMENTS BEING CONNECTED IN THE BRIDGE SO THAT THE VOLTAGE ACROSS THE BRIDGE PASSES IDENTICAL CURRENT THROUGH THE ELEMENTS WHOSE VALUE VARIES WITH VARIATION IN THE VOLTAGE ACROSS THE BRIDGE, SAID ELEMENTS EACH HAVING A SUBSTANTIAL TEMPERATURE COEFFICIENT OF RESISTANCE SO THAT THEIR RESISTANCES VARY WITH THEIR TEMPERATURE; MEANS FOR PLACING AN AIR SAMPLE IN CONTACT WITH BOTH OF SAID ELEMENTS UNDER SUBSTANTIALLY IDENTICAL CONDITIONS WHEREBY THE PRESENCE OF COMBUSTIBLE GAS IN THE SAMPLE WILL CAUSE CATALYTIC COMBUSTION AT THE SURFACE OF SAID ACTIVE ELEMENT TO RAISE ITS TEMPERATURE AND EFFECT A DIFFERENCE IN THE TEMPERATURE AND THUS THE RESISTANCE OF SAID ELEMENTS WHICH WILL CAUSE UNBALANCE OF THE BRIDGE; AND MEANS RE- 